1) Field of the Invention
The present invention relates to a polyolefin composition, a molded article thereof and a muliti-layered laminate, and more particularly to a polyolefin composition having well-balanted properties in which anisotropy in mechanical strength generating during molding-processing is small, furthermore great tearability and superior rigifiity, and to a molded article thereof, and also to a multilayered laminate being suitably used as a retort packaging material and excellent in tearability, rigidity, impact resitance and heat sealability.
2) Related Prior Art
Propylene resin is generally cheap and excellent in characteristics such as transparency, mechanical strength, heat resistance, gloss, chemical resistance, oil resistance, rigidity, flex fatigue resistance, etc., and is used in wide fields as various industrial materials and packaging materials.
When a propylene resin is molding-processed, its molecular chains are cooled to solidify in a stretched state toward a machine direction (hereinafter referred to as "M.D") by shear effects during molding-flowing, thereby generating remarkable anisotropy in properties. Generally, when orientation in a molded article is high, anisotropy in properties is generated in the molded article, whereby properties in a transverse direction to the flow (hereinafter referred to as "T.D") deteriorate.
Moreover, sometimes, heat resistance, weathering resistance, etc. deteriorate owing to interior strains due to molecular orientation. In order to improve it, various improving methods have been so far proposed and practically applied. For example, in extrusion molded films, there has been conducted secondary processing in which after molding biaxially stretched processing is conducted to keep the balance between M.D and T.D.
In injection molded articles and blow molded articles, there has been applied a method in which after molding a heat treatment is conducted.
However, the secondary processing method for films has such problems that not only a high cost for the processing is necessary, but also the use is limited since heat sealability deteriorates.
The method in which after molding a heat treatment is conducted provides complicated production steps and consequently has problems in economy and productivity.
Hitherto, as a packaging material for articles requiring a sterilizing treatment step, e.g, typically processed foodstuffs, a parcel that is called "retort pouch" has been used. In the Pouch, for long term-preservation, laminates having layers being excellent in gas barrier property have been used.
Recently, as a material of a pouch being excellent in heat resistance, cold resistance and impact resistance, there have been used multilayered laminates comprising the combination of block copolymer of polypropylene used in most inner layer, a polyester film and an aluminum foil, e.g., layers of polyester/aluminum foil/block copolymer of polypropylene.
However, the above-mentioned multilayered laminates have a drawback that tearability thereof is inferior, thereby causing a problem that the pouch cannot be easily teared when it is opened.
In attempts to improve tearability, it has been proposed to set a V-shaped or I-shaped cut, to provide unpierced sewing machine perforations, to conduct a near infrared ray treatment or an electric discharge treatment(JP-A 62-158648) for tear-expecting portions, etc.
However, these procedures have a problem that tearability is still insufficient, so that a considerable force for tear is required owing to resistance for tear.